Approximation bounds for a general class of precedence constrained parallel machine scheduling problems

An important class of scheduling problems concerns parallel machines and precedence constraints. We consider precedence delays, which associate with each precedence constraint a certain amount of time that must elapse between the completion and start times of the corresponding jobs. Together with ordinary precedence constraints, release dates and delivery times can be modeled in this manner. We present a 4-approximation algorithm for the total weighted completion time objective for this general class of problems. The algorithm is a rather simple form of list scheduling. The list is in order of job midpoints derived from a linear programming relaxation. Our analysis unifies and simplifies that of a number of special cases heretofore separately studied, while actually improving many of the former approximation results.